As a method for producing thermoplastic polyurethane, a solution polymerization method using a solvent and a bulk melt polymerization method not using a solvent have been known.
A method for producing a polyurethane elastomer by solution-polymerizing a polymer glycol, a diisocyanate compound and a crosslinking agent batchwise in a solvent gives a polyurethane elastomer having a relatively narrow molecular-weight distribution, but does not provide so high a degree of polymerization, requires a step of separating the solvent from the reaction product after the reaction, and is batchwise. Therefore, said method is extremely inefficient.
Japanese Patent Publication Sho 39-17098 (No. 17,093/1964) discloses a method for producing a polyurethane elastomer by bulk-polymerizing a polymer glycol and a diisocyanate compound batchwise in the absence of a solvent with a kneading machine such as a kneader.
This method affords a polyurethane elastomer having a high degree of polymerization. However, this polyurethane elastomer has a wide molecular-weight distribution and shows strong adhesion, so that said elastomer can hardly be peeled off from a rotating blade. In general, when conducting the reaction batchwise, it is very hard to strictly regulate proportions of starting materials to be fed in each batch, treatment temperatures, treatment times, a water content, extrusion times or the like. Due to this, a degree of polymerization and a molecular-weight distribution cannot be controlled and hence, it is quite difficult to make products uniform.
On the other hand, Japanese Patent Publication Sho 44-25600 (No. 25,600/1969) discloses a method for continuously producing a thermoplastic polyurethane resin, which comprises kneading a heat-molten liquid polyisocyanate compound and a compound containing at least two groups having an active hydrogen capable of reacting with an isocyanate group in a multi-screw extruder for a residence time of 3 to 60 minutes to polymerize them. This document describes 50.degree. to 200.degree. C. as a polymerization temperature, and employs a residence time of 7 to 15 minutes and a polymerization temperature (a cylinder temperature) of 140.degree. to 195.degree. C. in Examples.
Japanese Patent Publication Sho 49-3451 (No. 3,451/1974) discloses a method for producing polyurethane elastic yarns by a continuous synthesis in the absence of a solvent and a direct spinning, which method includes a step of preliminarily reacting a high-molecular dihydroxy compound having a molecular weight of 500 or more with an organic diisocyanate such that 70 to 100% of a finally necessary amount of the high-molecular dihydroxy compound substantially becomes an isocyanate group, Said document shows 150.degree. to 250.degree. C. as a reaction temperature of a prepolymer formed in the above preliminary reaction step, the remaining high-molecular dihydroxy compound and a low-molecular dihydroxy compound, and adopts 180.degree. C..times.15 minutes and 160.degree. C..times.20 minutes as reaction conditions in Examples.
Japanese Patent Publication Hei 3-54963 (No. 54,963/1991) discloses a method in which when continuously producing a thermoplastic polyurethane by reacting in an extruder a high-molecular diol having an average molecular weight of 500 to 3,000, a low-molecular diol having a molecular weight of less than 500 and an organic diisocyanate, a molar ratio of the amount of isocyanate groups of the organic diisocyanate to a total amount of hydroxyl groups of the high-molecular diol and the low-molecular diol is adjusted to the range of 1.+-.0.002 to control variation in melt viscosity of the resulting polyurethane. Said document describes that regarding the reaction (polymerization) temperature in the extruder, when the temperature is too high, the reaction rate becomes high and the mixing rate becomes low relatively, so that the mixing is insufficient and uniform polyurethane cannot be obtained or decomposition of the organic diisocyanate and the reaction product occurs, which is therefore undesirable. The recommanded polymerization temperature (a temperature inside the extruder) which is adopted in Examples of said document is 220.degree. C. at the highest. Said document does not have a description of 220.degree. C. or higher.
Japanese Laid-open Patent Application (Kokai) Hei 2-20514 (No. 20,514/1990) discloses a process for producing thermoplastic polyurethane by melt polymerization using, as a high-molecular diol, a polyester diol or a polycarbonate diol containing a residue of 2-methyl-1,8-octanediol or a residue of 1,9-nonanediol. Said document shows a polymerization temperature of at least 200.degree. C. but at most 240.degree. C., and describes that with the polymerization temperature of 240.degree. C. or lower, heat resistance of the resulting thermoplastic polyurethane increases.
Japanese Laid-open Patent Application (Kokai) Hei 2-191621 (No. 191,621/1990) discloses a thermoplastic polyurethane in which MB being MFR (a melt flow ratio) at 220.degree. C. is 5 to 50 and a ratio (MB/MA) of MB and MA being MFR a 200.degree. C. is 1 to 5. Said document states that the thermoplastic polyurethane has low temperature dependence of melt fluidity near 200.degree. C. and is uniform and that conventional polyurethane containing an undissolved material or gel in large amounts cannot achieve this value. It is described in this document that the undissolved material is Judged by dissolving a given amount of polyurethane in dimethylformamide at room temperature and measuring an amount (wt. %) of the undissolved material. Said document discloses Example in which the reaction (polymerization) of a prepolymer from a high-molecular dlol and an organic diisocyanate with a low-molecular diol was conducted at a fixed temperature of 200.degree. C. and Example in which the reaction was conducted at progressively elevated temperatures of 180.degree. to 200.degree. C.
The present inventors' investigations have revealed, however, that the polyurethane elastomer obtained by the above techniques, when subjected to ordinary melt-spinning, has difficulty in controlling the spinning temperature, allows formation of gels that prevent passage through a filter, and causes yarn breakage in spinning, etc. As a result, spinning for a long time is impossible, the yield varies in the range of 50 to 80%. Further, when producing a polyurethane elastomer sheet, a dimensional stability is inferior and fish eyes are formed, making it impossible to stably provide a good-quality sheet; moldability is still insufficient.